1. Field of the Invention
The present invention relates to an electric motor which is intended in particular for propeller-type fans, the rotor shaft of said electric motor being rotatably mounted in a plain bearing placed within a tubular support which is rigidly fixed to the motor frame.
2. Description of the Prior Art
Plain bearings of sintered alloy, for example, are particularly advantageous in small electric motors. They are in fact self-lubricating and are not only less costly to produce than ball-bearings but also provide quieter operation than these latter. It is for this reason that the use of plain bearings often proves advantageous, particularly in the case of electric motors which are employed for driving fans and the acoustic noise performances of which are of primary importance.
One plain bearing of known type which is particularly advantageous consists of a single cylindrical part which extends along the entire rotating portion of the rotor shaft, thereby eliminating the problems of alignment related to the use of two bearings in axially spaced relation.
In spite of these advantages, this type of bearing is still in relatively limited use since it suffers from practical difficulties, especially in regard to immobilization of the bearing in its support. It is in fact usually forcibly fitted within a tubular support rigidly fixed to the motor frame or even directly within an axial bore formed in said frame. By reason of this intimate contact between the bearing and the motor frame, the shocks to which the frame is subjected, for example as a result of imbalance of the rotor, are transmitted to the bearing which behaves as an amplifier and produces vibrations. This is particularly true in the case of a rotor which is rigidly fixed to the propeller. In some instances, this phenomenon may acquire such preponderance as to nullify the advantages of a plain bearing over ball-bearings in regard to acoustic-noise performances.
Furthermore, the sintered alloy which constitutes this type of bearing is relatively brittle and is consequently liable to suffer degradation as a result of overheating which essentially arises from the operation of the motor or from mechanical stresses related to faulty centering of the rotor shaft.